Investigation of Xylose Reversion Reactions That Can Occur during Dilute Acid Pretreatment

Xylose reversion reactions to form xylooligomers represent a potentially important mechanism of sugar loss during dilute acid pretreatment of biomass. We have conducted a study to identify the products that result from these reactions and to determine the kinetics of their formation. A major obstacle is that there are few commercial standards available for xylose disaccharides, which are essential for the identification and quantification of the xylose reversion products formed during these reactions. To overcome this obstacle, we have used GC/MS and NMR analysis of xylose disaccharides isolated by preparative HPLC to identify the reaction products. At the xylose concentration we used (300 g L-1), only xylose disaccharides were observed. As with glucose reversion reactions [Pilath, H. M.; et al. J. Agric. Food Chem. 2010, 58, 6131], the disaccharides contained linkages that involved the anomeric carbon atom of one of the sugar monomers. Eight out of the nine possible disaccharides, including alpha and beta anomers, were observed. Whereas the GC/MS allowed for the identification of the linkages, NMR was needed to distinguish between the alpha and beta isomers of the disaccharides. The kinetics of combined xylose disaccharide formation was measured using HPLC. Arrhenius parameters for the rates of disaccharide formation were calculated by fitting the data to a simple model.